Sometimes the mouth of the extractor does not correspond to the size of the filter or tube of the same diameter and reductions are needed. For that we use the reduction coupling in the appropriate measure.
It is also used for Y or T connections, to take two tubes out of one, since it is convenient to reduce the diameter of the tube.
When there are several bifurcations in a tube, only with step reductions can it be controlled so that it absorbs the same from one place as from another, for example, removing two 150mm or 125mm tubes from a 200 tube, since if you continue with two of 200mm, the air can suck it only from 1 tube and leave the other unusable, thus affecting the ventilation of the crop.
If we put a reduction, but on the contrary, to make an enlargement, for example from 125mm to 150mm at the exhaust air outlet, we will achieve that it has much less pressure when going out and makes less noise.